The present invention generally relates to treatment fluids containing biodegradable chelating agents, and, more particularly, to methods for treating at least a portion of a subterranean formation using treatment fluids containing a biodegradable chelating agent in conjunction with a hydrofluoric acid treatment.
Treatment fluids can be used in a variety of subterranean treatment operations. Such treatment operations can include, without limitation, drilling operations, stimulation operations, production operations, and sand control treatments. As used herein, the terms “treat,” “treatment” and “treating” refer to any subterranean operation that uses a fluid in conjunction with a desired function and/or for a desired purpose. Use of these terms does not imply any particular action by the treatment fluid. Illustrative treatment operations can include, for example, fracturing operations, gravel packing operations, acidizing treatments, scale dissolution and removal, consolidation treatments, and the like.
In acidizing treatments, for example, subterranean formations comprising acid-soluble components, such as those present in carbonate and sandstone formations, are contacted with a treatment fluid comprising an acid. After acidization is completed, the water and salts dissolved therein may be recovered by producing them to the surface, e.g., “flowing back” the well, leaving a desirable amount of voids or conductive pathways (e.g., wormholes in carbonates) within the formation, which enhance the formation's permeability and may increase the rate at which hydrocarbons may subsequently be produced from the formation.
Acidizing a siliceous formation should be distinguished from acidizing a carbonate formation. Carbonate formations can be treated with a variety of acid systems, including hydrochloric, acetic and formic acids, often with similar success. The treatment of siliceous formations with these acids, however, may have little or no effect because they do not react appreciably with the silica and silicates that characterize siliceous formations. As used herein the term “siliceous” refers to the characteristic of having silica and/or silicate. Most sandstone formations are composed of over about 40% to about 98% sand quartz particles, i.e., silica (SiO2) bonded together by various amounts of cementing material including carbonate (calcite or CaCO3), aluminosilicates, and silicates.
By far the most common method of treating sandstone formations involves introducing corrosive, very low pH acids comprising hydrofluoric acid into the well bore and allowing the acid to react with the surrounding formation. Hydrofluoric acid is very reactive with aluminosilicates and silicates. Hydrochloric acid may be used to maintain a low pH as hydrofluoric acid spends, retaining certain dissolved species in solution. The silicates include clays and feldspars. Hydrofluoric acidizing is often used to remove damage within the formation. Such treatments are generally not considered “stimulating” in the sense of creating or extending fractures in the formation as in a typical fracturing operation. As a result of a hydrofluoric acid treatment, it is desirable that the skin value in the formation be zero. It is not anticipated that it will be less than zero. Any damage left behind gives a positive skin value, which is not desirable.
Hydrofluoric acid can interact with the formation, fluids, or other fluids present therein to create precipitates, which leads to damage and a positive skin value. For instance, hydrofluoric acid tends to react very quickly with authigenic clays, such as smectite, kaolinite, illite and chlorite, especially at temperatures above 150° F., as a function of mineral surface area. Because of this quick reaction, acid may penetrate only a few inches into the formation before the hydrofluoric acid is spent. Simultaneously, precipitation of various aluminum and silicon complexes occur as a result of the reaction of the acid with the siliceous minerals. Damage to the formation may result from this precipitation. At certain temperatures and subterranean conditions dissolution of the sandstone matrix may occur so rapidly that uncontrolled precipitation can become an inevitable problem. The precipitation products plug pore spaces and reduce the porosity and permeability of the formation, thus impairing flow potential.
Because clays are normally a part of the cementitious material that holds the sandgrains of siliceous formations together, the dissolution of clay also weakens and de-consolidates the sandstone matrix in the vicinity of the well bore, thus causing damage to the formation. Any metal ion has the potential to create problems if not adequately managed. The damaging effects due to both the de-consolidation of the matrix and the precipitation of complexes which clog the pore spaces of the formation can eliminate or even revert the stimulation effect of the acid treatment.
Of particular concern is the formation of calcium fluoride, fluorosilicates, or other insoluble fluoro-compounds, which can negate the effectiveness of a hydrofluoric acid treatment and cause damage to the formation. This can lead to production delays while damage control operations are conducted. The fluorosilicates can be particularly problematic because they are the primary product of the dissolution of a clay and hydrofluoric acid. Fluorosilicates are difficult to remediate. Calcium fluoride can be a later concern in the process because the fluoride anion needs to be present, in its free ion form, and that does not happen until a higher pH is reached. Calcium fluoride can be remediated, in some instances. Remediation techniques include a commercially available treatment system from Halliburton Energy Services, Inc. known as “F-SOL™” acid system is used to dissolve calcium fluoride). Another source of concern is the production of fluoro-aluminates as a consequence of the reaction of fluorosilicates with clay minerals. These fluoro-aluminates are thought to be soluble as long as the pH is below 2 and the ratio of F/Al is maintained below 2.5. If precipitated, their dissolution requires strong HCl (>5%).
Avoiding the formation of calcium fluoride, fluorosilicates, or other insoluble fluoro-compounds can be a primary design objective. Various means have been used with mixed success. Blends of organic acids and hydrofluoric acid have been used to slow the dissolution kinetics of sandstone formation solids. However, as organic acids have higher pKa values than do mineral acids, precipitation can become problematic as the treatment fluid pH rises. Pre-flush sequences with acids have been used to remove calcium salts from sandstone formations, before the main acidizing sequence is conducted to remove formation aluminosilicates. Generally, these flushes do not spend completely and typically return, upon flowback, with a persisting low pH. This can result in corrosion of downhole tubular goods (including coiled tubing) and surface equipment. Other multi-stage sandstone acidizing treatment operations have also been developed, particularly to remove calcium ions. Chelating agents can also be included in treatment fluids to sequester at least a portion of the formation cations that cause unwanted precipitation, however, there are certain operational problems that are encountered with use of many commonly used chelating agents. First, many commonly used chelating agents are not biodegradable or present other toxicity concerns that make their use in a subterranean formation problematic. Further, the salt form of some chelating agents can actually exacerbate precipitation problems in a hydrofluoric acidizing treatment rather than lessening the amount of precipitated solid.